Hybrid resistance is the prevention of proliferation of grafts of H-2 homozygous normal or neoplastic hemopoietic cells by F1 hybrid mice heterozygous at H-2. This observation goes against the laws of transplantation genetics, which state that histocompatibility antigens are inherited in a codominant fashion. The hybrid or hemopoietic histocompatibility (Hh) antigens recognized have not been characterized chemically even though the determinants map at H-2D. One aim here is to generate monoclonal antibodies to Hh antigens. The effector cells responsible for hybrid resistance include natural killer cells (NK), which probably function in surveillance against tumors. Cells other than NK cells are also probably involved and may have the specific receptors for Hh antigens. A second aim is to generate monoclonal antibodies against the effector cells involved in hybrid resistance responses. To enhance the success of the above two aims, we aim to develop an in vitro assay for hybrid resistance. The potential importance of the work may relate to antiself immunity (autoimmunity and tumor surveillance), since the parent and the F1 share the structural genes for the same Hh antigen. (SR)